This invention relates to an ignition timing control apparatus for an internal combustion engine, and in particular to an ignition timing control apparatus for an internal combustion engine which is operable either with a low-octane rating fuel or a high-octane rating fuel.
It is well known in the art that the octane rating of fuel gasoline is greatly interrelated with a knock-resistibility in an internal combustion engine. Namely, as the octane rating of gasoline becomes higher, the knocking of an engine correspondingly decreases.
FIG. 1 shows an ignition timing vs output shaft torque characteristic in an internal combustion engine in the case where a commercially available low-octane gasoline (regular gasoline) and a commercially available high octane gasoline (premium gasoline) of which the octane rating is higher than that of the low-octane gasoline. In the figure, point A indicates a knock limit for the low-octane gasoline while point B indicates a knock limit for the high-octane gasoline, in which knock arises when the angle of the ignition timing is advanced beyond these points. According to FIG. 1, it is possible to advance the angle of the ignition timing up to a point B when using the high-octane gasoline, where the output shaft torque will be higher as compared with the use of the low-octane gasoline.
FIG. 2 shows an ignition timing characteristic representing points A and B as a function of engine speed. It is seen that the interval between the characteristics A and B is constant as a function of the engine speed. Therefore, in an internal combustion engine with such a characteristic, it will be possible to increase the output of the engine by advancing the ignition timing by a predetermined angle if a fuel used in the engine is changed over from the low-octane rating gasoline to the high-octane rating gasoline.
However, in a conventional ignition timing control apparatus for an internal combustion engine, the reference ignition timing characteristic is preset only for a predetermined gasoline, e.g. the low-octane rating gasoline, and therefore when using the high-octane rating gasoline, an increase in the output of the engine can not be expected without any change or modification thereof, so that the presetting of the reference ignition timing must be repeated towards the advance angle by some method.
Also if the ignition timing is set in the direction of the advance angle adapted for the premium gasoline, disadvantageous events may occur at the starting time of engine or in other operating regions.
U.S. Pat. No. 3,822,583 issued to Keller et al. on July 9, 1984 discloses "Method for Determining Octane Ratings of Fuels under Road Conditions" in which knocking is detected to measure the octane rating of fuels, thereby controlling the ignition timing.
U.S. Pat. No. 3,718,126 issued to Oishi et al. on Feb. 27, 1973 discloses "Ignition Timing Regulating Device for Internal Combustion Engine" in which the rotating speed, load, etc. of an engine is detected to phase-shift the ignition timing.